Various embodiments disclosed herein relate generally to optical elements, security liquid crystal cells and methods of making the same.
Display screens on mobile devices, ATM's, and other machines that may be used outdoors often have problems with sunlight readability, UV degradation, durability, operating temperature range, and lifetime. Sunlight readability may be improved in a number of ways. One solution is to actively increase the backlight intensity by adding more cold-cathode-fluorescent-lamp (CCFL) backlight tubes. Unfortunately, this approach has drawbacks in most mobile device applications because of battery drain, larger device size, heat generation, and weight considerations. A second approach is to passively increase backlight intensity by adding brightness-enhancement films to the optical stack of the LCD. While avoiding most of the drawbacks of the active approach, this solution only increases brightness by a factor of about two, which is insufficient to solve the sunlight readability problem. A third solution is the minimization of reflected light, such as through the use of anti-reflective coatings and films and circular polarizers. Each of these solutions may be combined with others to optimize the desired effect.
A circular polarizer is an assembly of a conventional linearly polarizing element and a quarter wave retarder. The axis of the retarder is oriented at 45 degrees with respect to the axis of the linear polarizer. As incident light passes through the assembly, it is converted to circularly polarized light. Circular polarizers have traditionally been used for their antireflective properties. In such applications, when light is reflected back from a specular surface through the retarder, the plane of polarization is rotated 90 degrees with respect to the original orientation so the linear polarizer blocks the returning reflected light.
Conventional linearly polarizing elements, such as linearly polarizing lenses for sunglasses and linearly polarizing filters, are typically formed from stretched polymer sheets containing a dichroic material, such as a dichroic dye. The conventional linearly polarizing elements are static elements having a single, linearly polarizing state. Thus when a conventional linearly polarizing element is exposed to either randomly polarized radiation or reflected radiation of the appropriate wavelength, some percentage of the radiation transmitted through the element will be linearly polarized. As used herein the term “linearly polarize” means to confine the vibrations of the electric vector of light waves to one direction or plane.
Further, conventional linearly polarizing elements are typically tinted using a coloring agent (i.e., the dichroic material) and have an absorption spectrum that does not vary in response to actinic radiation. As used herein “actinic radiation” means electromagnetic radiation, such as but not limited to ultraviolet and visible radiation that is capable of causing a response. The color of the conventional linearly polarizing element will depend upon the coloring agent used to form the element, and most commonly is a neutral color (for example, brown or gray). While conventional linearly polarizing elements are useful in reducing reflected light glare, because of their tint they are not well suited for use under certain low-light conditions. Further, because conventional linearly polarizing elements have only a single, tinted linearly polarizing state, they are limited in their ability to store or display information.
As discussed above, conventional linearly polarizing elements are typically formed using sheets of stretched polymer films containing a dichroic material. As used herein the term “dichroic” means capable of absorbing one of two orthogonal plane polarized components of transmitted radiation more strongly than the other. Thus, while dichroic materials are capable of preferentially absorbing one of two orthogonal plane polarized components of transmitted radiation, if the molecules of the dichroic material are not suitably positioned or arranged, no net linear polarization of transmitted radiation will be achieved. That is, due to the random positioning of the molecules of the dichroic material, selective absorption by the individual molecules will cancel each other such that no net or overall linear polarizing effect is achieved. Thus, it is generally necessary to suitably position or arrange the molecules of the dichroic material by alignment with another material in order to achieve a net linear polarization.
One common method of aligning the molecules of a dichroic dye involves heating a sheet or layer of polyvinyl alcohol (“PVA”) to soften the PVA and then stretching the sheet to orient the PVA polymer chains. Then the dichroic dye is impregnated into the stretched sheet and dye molecules take on the orientation of the polymer chains. That is, the dye molecules become aligned such that the long axis of the dye molecule are generally parallel to the oriented polymer chains. Alternatively, the dichroic dye can be first impregnated into the PVA sheet, and then the sheet can be heated and stretched as described above to orient the PVA polymer chains and associated dye. This allows the molecules of the dichroic dye to be suitably positioned or arranged within the oriented polymer chains of the PVA sheet and a net linear polarization to be achieved. That is, the PVA sheet can be made to linearly polarize transmitted radiation, or in other words, a linearly polarizing filter can be formed.
In contrast to the dichroic elements discussed above, conventional photochromic elements, such as photochromic lenses that are formed using conventional thermally reversible photochromic materials are generally capable of converting from a first state, for example a “clear state,” to a second state, for example a “colored state,” in response to actinic radiation, and reverting back to the first state in response to thermal energy. As used herein the term “photochromic” means having an absorption spectrum for at least visible radiation that varies in response to at least actinic radiation. Thus, conventional photochromic elements are generally well suited for use in both low-light and bright conditions. However, conventional photochromic elements that do not include linearly polarizing filters are generally not adapted to linearly polarize radiation. That is, the absorption ratio of conventional photochromic elements, in either state, is generally less than two. As used herein the term “absorption ratio” refers to the ratio of the absorbance of radiation linearly polarized in a first plane to the absorbance of the same wavelength radiation linearly polarized in a plane orthogonal to the first plane, wherein the first plane is taken as the plane with the highest absorbance. Therefore, conventional photochromic elements cannot reduce reflected light glare to the same extent as conventional linearly polarizing elements. Further, conventional photochromic elements have a limited ability to store or display information.
Accordingly, it would be advantageous to provide elements and devices that are adapted to display both linearly polarizing and photochromic properties. Further, it would be advantageous to provide elements and devices that are adapted to display circular or elliptical polarization and photochromic properties, for example, in an effort to improve sunlight readability of display screens. Such elements and devices can also be used to improve visibility through packaging materials and protect light-sensitive items contained within the packaging materials.